The invention relates to time division multiplex (TDM) telecommunication switching systems, more particularly pulse code modulation (PCM) telephone switching systems, having a TDM ring-like circuit running from subscriber to subscriber.
TDM telephone switching systems of this type are old in the art (cf. U.S. Pat. No. 3,586,782). In such a telephone switching system, connections may be established such that a plurality of time slots are defined by a central timing station connected in the ring-like circuit, wherein each subscriber station is synchronized to the frame of the time slots by a synchronizing signal emitted by the timing station in a synchronizing time slot. For the establishment of a connection the calling subscriber station uses a time slot identified as free by the timing station and sends in that time slot the signal of the called subscriber station. All currently inoperative subscriber stations supervise all time slots as the reception of their signals and, where necessary, are switched to the connection state in the time slot in which they receive (or have received) their signals.
In telecommunication switching systems, more particularly telephone switching systems, there is frequently the requirement that, in addition to connections between two subscribers, calls known as conference or cut-in calls also be set up between more than two subscribers. Conference calls can simply be set up in switching systems in which the speech information is transferred by means of analog signals by interconnecting the subscriber lines of the subscribers involved directly or via buffer storages. This results in a mixing of the speech signals of all the conferees (cf. U.S. Pat. Nos. 3,293,369 and 3,319,005).
In switching systems operating according to the pulse-code-modulation principle, conference calls cannot readily be set up by mixing the various speech signals in view of the generally non-linear modulation characteristic. To conduct conference calls in a PCM telephone switching system, it is known (CCITT documents WP 33/XV No. 6-E of August 1965, page 2, item (3) and WP 33/XV No. 9-E of December 1965, page 10, second paragraph) to convert the PCM signals obtained from the speech signal samples of the parties through non-linear coding, that is, in accordance with a non-linear modulation characteristic, to a linear code, or to "linearize them digitally". The "linear" PCM signals are added in binary fashion and thereafter the composite signals, so obtained, are reconverted to the non-linear code.
A method equivalent to this known method is also found in U.s. Pat. No. 3,575,591. It is likewise known (U.S. Pat. No. 3,575,591) to store temporarily the "non-linear" PCM signals in first registers, convert them to the linear code one after the other by means of a common converter, to store the "linear" PCM signals in second registers, thereafter to add them in an accumulator and, finally, to convert the composite signals obtained to "non-linear" PCM signals with the aid of the converter. U.S. Pat. No. 3,575,591 further recites the routing to each subscriber in a three-party conference circuit of only the composite signals obtained by adding the PCM signals of the other two subscribers. The known conference circuit proceeds from a concentrator switching system to which the individual subscriber stations are connected. It is not suited for exchangeless digital telecommunication systems having a ring-like circuit running from subscriber station to subscriber station.
It is an object of this invention to provide a method for connecting in a conference call at least three subscribers to a TDM telecommunication switching system having a TDM ring-like circuit running from subscriber station to subscriber station.